1. Field of the Invention
The present invention relates generally to processes for making low density, porous silicon dioxide films and more particularly relates to processes for making porous silica films for use as an insulating material for integrated circuits.
2. Description of Related Art
Advanced integrated circuits for semiconductor devices having higher performance and greater functionality are often characterized by decreasing device feature geometries. As device geometries become smaller, the dielectric constant of an insulating material used between conducting paths becomes an increasingly important factor in device performance. Reducing this value advantageously lowers power consumption, reduces crosstalk, and shortens signal delay for closely spaced conductors.
Silicon oxide (SiO.sub.2) has long been used in integrated circuits as the primary insulating material. With a dielectric constant of approximately 4, SiO.sub.2 has the lowest dielectric constant of all inorganic materials. One option for achieving a dielectric material with a lower dielectric constant is to use organic or polymeric materials However, these materials tend to have limited thermal and mechanical stability.
Another approach is to use porous, low density materials in which a significant fraction of the bulk volume contains air, which has a dielectric constant of approximately 1. The properties of porous materials are proportional to their porosity. For example, at a porosity of about 80%, the dielectric constant of a porous silica film, i.e. porous SiO.sub.2, is approximately 1.5. The potential utility of porous silica, particularly silica aerogels, as a low dielectric constant insulating material has been recognized. (See, for example, L. Hrubesh, Mat. Res. Soc. Symp. Proc. Vol. 381, p. 267 (1995) and M. Jo et al., Microelectronic Engineering, Vol. 33, p. 343 (1997).)
Aerogels are sol-gel derived solids consisting of molecular sized clusters which themselves have microporosity, and which are connected in such a way that a three dimensional structure forms without having pores larger than a few cluster diameters. The rigid skeleton occupies only a very small fraction of the total volume. A typical sol-gel process for applying a silica aerogel thin film on a semiconductor substrate is described, for example, in U.S. Pat. No. 5,569,058, "LOW DENSITY, HIGH POROSITY MATERIAL AS GATE DIELECTRIC FOR FIELD EMISSION DEVICE." A precursor solution of tetraethylorthosilicate (TEOS), ethanol, water, and small amounts of HCl and NH.sub.4 OH is applied to the substrate surface. The precursor solution is gelled on the surface, a process which takes up to 12 hours, and aged in a saturated ethanol atmosphere for approximately 24 hours at 37.degree. C. To prevent premature drying, the surface is immersed in liquid or in a saturated atmosphere at all times prior to the drying stage. Deposition of the precursor solution on silicon substrates using conventional spinning and dipping methods of semiconductor fabrication are described, for example by Hrubesh and references therein. The methods are performed in enclosures maintained with a saturated alcohol atmosphere.
The next step in the sol-gel process is to dry the alcohol-filled gel without collapsing the structure or inducing shrinkage and densification. A typical method of drying uses supercritical extraction. Unfortunately, supercritical drying requires high pressure and is therefore difficult to accomplish in a high-production environment. Also, supercritically-dried aerogel tends to be hydrophilic. Alternative methods involving additional steps of solvent exchange and surface modification and drying at sub-critical or supercritical pressures are described in U.S. Pat. No. 5,470,802, "METHOD OF MAKING A SEMICONDUCTOR DEVICE USING A LOW DIELECTRIC CONSTANT MATERIAL." Aerogels produced by sol-gel processes typically have pore diameters less than 50 nm.
While thin film silica aerogels for use as dielectric layers have been prepared by sol-gel techniques, these techniques are not well adapted for high-throughput semiconductor processing environments. The sol-gel process is a wet chemistry process, requiring long processing times, saturated alcohol atmospheres, and, in many applications, high pressure for supercritical solvent extraction. There are problems associated with the wet chemistry sol-gel process of controlling particulate contamination and of controlling pore size and film shrinkage on drying. Thus, it would be desirable to provide a method of producing a low dielectric constant low density, porous silica film in a dry, vacuum environment that avoids the wet chemistry processing steps of the sol-gel technique. It would be desirable if the method avoids the problem of film shrinkage and if the porous silica film produced is more hydrophobic than sol-gel produced silica aerogel films.